There are known two general methods of converting an alternating voltage into a direct voltage. Method 1: full wave rectification is implemented on an alternating current from an alternating voltage power supply using a diode bridge circuit, and the direct current subjected to the full wave rectification is smoothed using a smoothing capacitor.
In method 1, generally, if an alternating voltage is positive or negative, the current flows through a series circuit composed of two diodes, resulting in a power loss which is equivalent to the product of the current flowing both the diodes and the forward voltage of the diodes.
Method 2: a power factor improvement converter (PFC) is configured between the diode bridge circuit and the smoothing capacitor used in method 1. The power factor improvement converter carries out a control to change the current flowing to the alternating voltage power supply into a sine-wave current and a control to equalize a sine-wave current to the voltage phase of the alternating voltage power supply.
In method 2, the current also flows through a series circuit composed of two diodes during a full wave rectification process, thus causing a power loss. Moreover, a current alternatively flows through a Field Effect Transistor (FET) which constitutes the power factor improvement converter and the diodes, leading to a further loss.
Further, apart from being required to change the waveform of an input current into a sine wave, the power factor improvement converter is also required to set an output voltage to be higher than the input voltage. However, the voltage needed by a load is not definitely higher than the input voltage. In this case, a buck converter is connected with and located at the rear side of the power factor improvement converter so that the voltage stepped up by the power factor improvement converter is reduced to a desired voltage. A loss is caused by the voltage reduction. A whole power converter consists of three sections: an AC-DC converter, a DC-DC (step-up) converter and a DC-DC (step-down) converter, and the power conversion efficiency of the power converter is presented as the product of the conversion efficiencies of the three sections. For example, if the efficiency of each section is 0.95, then the product of the three sections is: 0.95*0.95*0.95=0.86. In other words, even if each section achieves an outstanding efficiency of up to 95%, the whole conversion efficiency drops to 86%. Thus, a power converter, if composed of a plurality of sections, is obviously reduced in conversion efficiency even if the conversion efficiency of each section is remarkable.
At present, in addition to the requirement on being more power-saving, no current harmonic noises for external environment becomes another necessary requirement on electronic machines. Thus, it is needed to improve the conversion efficiency of a power converter which supplies the power to a load and endow the power converter with a current harmonic suppression function.